1. Field of the Invention
This invention relates to sintering agents to be used with silicon nitride to produce a sintered product having a refractory oxide phase with good strength at low cost.
2. Description of the Previously Published Art
Sintering agents are typically necessary to permit the silicon nitride particles to sinter together to form a dense material.
GTE Laboratories in U.S. Pat. Nos. 4,431,431; 4,433,979; 4,440,547; 4,441,894; 4,449,989; 4,497,228; and 4,650,498 disclose densified composite ceramic articles where there are two phases of silicon nitride with a second phase having a densification aid of the oxides of silicon, aluminum, magnesium, zirconium, yttrium, hafnium, cerium, and the lanthanide rare earths.
U.S. Pat. No. 4,440,707 discloses a silicon nitride product sintered one or more components of AlN, Al.sub.2 O.sub.3, SiO.sub.2 and oxides of rare earth aids.
U.S. Pat. No. 4,511,402 discloses a silicon nitride sintered body having 0.1-15% by weight of one or more chromium components and 1 to 25% by weight of one or more oxides of Sc, Y, Al, Zr and Si.
U.S. Pat. No. 4,535,063 discloses a silicon nitride sintered body containing specific amounts of oxides or oxynitrides of Sr, Mg, a rare earth element and Zr with the remainder being Si.sub.3 N.sub.4.
U.S. Pat. No. 4,795,724 discloses silicon nitride sintered bodies having a sintering aid which has at least two rare earth elements from the group Y, Er, Tm, Yb and Lu. In most of the examples the expensive element Y is present in substantial amounts. Examples without Y have combinations of Er and Yb or Yb and Lu.
U.S. Pat. No. 4,801,565 discloses a silicon nitride sintered body containing at least an oxide of a rare earth element and ZrO.sub.2.
U.S. Pat. No. 4,810,678 discloses a silicon nitride base sintered body where the sintering aid is Y.sub.2 O.sub.3 in combination with Nd.sub.2 O.sub.3 and/or Sm.sub.2 O.sub.3. Substantial amounts of expensive Y.sub.2 O.sub.3 are used and most of the examples illustrate a two component sintering aid of Y.sub.2 O.sub.3 and Nd.sub.2 O.sub.3.
U.S. Pat. No. 4,830,800 discloses a dense silicon nitride compact having Al.sub.2 O.sub.3 or AlN, and a rare earth oxide or a rare earth nitride. The preferred rare earth appears to be Y.
U.S. Pat. No. 4,830,991 discloses a silicon nitride sintered body containing an intergranular phase being substantially crystallized into diopside structural type crystals and apatite structure type crystals. A composition including Ca, Y, a rare earth element, Mg, Fe, Ni and/or Al is formulated into a Si.sub.3 N.sub.4 raw material.
3. Objects of the Invention
It is an object of this invention to obtain high sintered densities of silicon nitride without employing the commonly used expensive additive Y.sub.2 O.sub.3.
It is a further object of this invention to obtain flexural (4 point) properties of 80 Ksi and above.
It is a further object of this invention to obtain sintered high density silicon nitride products.
It is a further object of this invention to obtain a refractory product which is believed to have high melting rare earth silicate phases.
It is a further object of this invention to obtain an alpha to beta conversion of higher than 75% in the absence of yttria and preferably higher than 80%.
These and further objects will become apparent as the description of the invention proceeds.